1. Field of the Invention
The present invention relates to a starter for an internal combustion engine of an automobile.
2. Description of the Prior Art
Starters for an internal combustion engine generally comprise a D.C. electric motor, an output shaft coupled to the electric motor, and a hollow cylindrical assembly carrying the pinion and mounted coaxially on the output shaft. This pinion-carrying hollow cylindrical assembly is slid axially forward and is driven by the output shaft when the engine is to be started. In this front position of the pinion-carrying hollow cylindrical assembly, the pinion engages and drives the ring gear of the associated internal combustion engine.
FIG. 1 shows a typical structure of a conventional starter in which the output shaft forms an integral extension of the armature shaft of the electric motor. The armature shaft 1a of the D.C. electric motor 1 is extended forward to form an output shaft 2 integral therewith. The hollow cylindrical pinion carrying assembly mounted coaxially around the output shaft 2 includes a overrunning clutch 3 and a hollow shaft 4 having a pinion 5 formed at the front end thereof. The rear extension 3d of the outer member 3a of the clutch 3 engages with the helical splines 2a formed on the enlarged diameter portion of the output shaft 2 at the keyways formed on the inner surface thereof. The inner member 3b of the clutch 3 driven by the outer member 3a through rollers 3c forms an integral rear portion of the pinion-carrying hollow shaft 4, which is rotatably and axially slidably supported by sleeve bearings 6 on the output shaft 2. The hollow shaft 4 is further rotatably and axially slidably supported by a ball bearing 7 within the front frame 8. An electromagnetic switch device 9, when activated, rotates a shift lever 10 through a plunger rod 9a to slide the clutch 3 forward together with the pinion-carrying hollow shaft 4 on the output shaft 2 so that the pinion 5 is engaged with the ring gear (not shown) of the engine. The forward movement of the pinion-carrying shaft 4 is stopped and limited by a ring-shaped stopper 11 fixedly secured to the front end of the output shaft 2 by means of a ring 11a fitted into a groove 2b formed on the output shaft 2.
FIG. 2 shows another typical structure of a conventional starter which is similar to that shown in FIG. 1. In the case of the starter of FIG. 2, however, the output shaft 2 is a separate member which is coupled to the armature shaft 1a of the electric motor through the intermediary of a planetary reduction gear train (not shown). Otherwise, the starter of FIG. 2 is similar to that of FIG. 1, like reference numerals representing similar parts or portions.
The conventional starters as described above suffer disadvantages because of the structure in which the forward movement of the pinion-carrying hollow shaft 4 is limited by the stopper 11 which is mounted to the end portion of the output shaft 2 by means of the ring 11a fitted into the groove 2b. Namely, this structure requires that the output shaft 2 include an additional length for the disposition of the stopper 11, and this additional length of the output shaft 2 increases the overall dimension of the starter. Further, the disposition of the stopper 11 increases the number of parts and steps necessary for assembling the starter, which results in increased production cost thereof.
In view of these disadvantages of conventional starters, a structure shown in FIG. 3 has already been proposed to improve the starter of FIG. 1 in which the output shaft forms an integral extension of the armature shaft of the electric motor. The starter of FIG. 3 has a structure similar to that shown at FIG. 1 (with exceptions to be described below), like reference numerals representing like parts or portions. In the starter of FIG. 3, however, the forward movement of the pinion-carrying hollow shaft 4 is stopped and limited by an annular flange 4a formed at a rear portion thereof. That is, the flange 4a abuts against the bearing 7 to stop the forward sliding movement of the hollow shaft 4 when the electromagnetic switch device 9 is activated and the rotation of the shift lever 10 slides the clutch 3 and the hollow shaft 4 to the front position. Thus, the stopper 11 of FIG. 1 can be dispensed with, and above-mentioned disadvantages of conventional starters can be eliminated.
The structure according to FIG. 3, however, suffers another kind of problem, as described below.
As described above, when the engine is started, the electromagnetic switch 9 is activated to turn the lever 10, and the pinion-carrying hollow shaft 4 is slid forward together with the clutch 3, so that the flange 4a abuts against the bearing 7 and the pinion 5 engages with the ring gear (not shown) of the engine. At the same time, the electric motor 1 is activated to rotate the pinion-carrying hollow cylindrical assembly. Thus, the rotation of the output shaft 2 is transmitted to the outer member 3a of the clutch 3 through the rear extension 3d thereof engaging with the helical splines 2a formed on the output shaft 2. Further, the rotation of the outer member 3a of the clutch 3 is transmitted through the rollers 3c to the rear extension 3b of the hollow shaft forming the inner member of the clutch 3. In this transmission of the rotational force from the electric motor to the pinion engaging with the ring gear of the associated engine, due to the fact that the rotation of the output shaft 2 is transmitted to the clutch 3 by means of helical splines 2a, the rotational force of the output shaft 2 not only results in a rotational force on the clutch 3, but also results in an axial force which drives the clutch 3 (and the pinion carrying hollow shaft 4 together therewith) axially forward. This axial driving force is born by the flange 4a abutting against the bearing 7. The reaction from the bearing 7 transmitted through the hollow shaft 4 and the clutch 3 acts on the output shaft 2 at the helical splines 2a, and drives the armature shaft 1a in the axially backward direction. Thus, the rear end surface of the commutator 1b of the motor 1 is pressed against the ring-shaped washer 12 by this backward reaction which is equal to the axial driving force arising at the helical splines 2a. The friction between the commutator 1b and the washer 12 brings about a loss of output torque which is equal to: (the diameter of the washer 12).times.(the axial force arising at the helical splines 2a).times.(the coefficient of friction between the commutator 1b and the washer 12). This torque loss substantially diminishes the output power of the starter.
In addition to the disadvantages described above, conventional starters all have the following disadvantage. Namely, since the outer surface of the pinion-carrying hollow shaft 4 is rotatably and axially slidably supported by the bearing 7, the hollow shaft 4 (and hence the output shaft 2) requires an additional axial length for the bearing 7, thereby increasing the size and weight of the starter.